1. Field of the Invention
The present invention relates to an improved high speed catadioptric objective lens system with a front lens, a main mirror lens, a counter mirror and a field lens system.
2. Description of the Prior Art
Heretofore known catadioptric objective lenses are subdivided into two fundamentally different classes of performance with different relative aperture ratio (speed). The objectives of the first mentioned class are the tele-lenses, used in photography. These have a speed of F/5 or less in contrast to the object of the present invention, which is high speed and belongs to the second mentioned class. An example of a tele-lens of the first class with a speed of F/7.4 is disclosed in U.S. Pat. No. 4,188,091.
An example of the class of high speed lenses is disclosed in Swiss Pat. No. 542,454 and by L. Canzek: "Lichtstarkes katadioptrisches Objektiv", Optica Acta, No. 12, 1971 and No. 4, 1972. The focus of this system is situated within the objective, impeding general use.
Other high speed objectives with the focus placed outside of the lens system are disclosed in L. Canzek: "Neue Richtung in der Entwicklung der katadioptrischen Objektive", Optica Acta, No. 2, 1979, and described in my co-pending U.S. application Ser. No. 967,973 filed Aug. 12, 1978, now U.S. Pat. No. 4,273,425. The known high speed objectives, however, have either a considerable length or they are characterized by rather large sphero-chromatic aberrations over a broad spectral range.
The sphero-chromatic aberrations, known also as sphero-chromatism, essentially impair the image quality. The effect of these aberrations is all the more harmful with objectives of large focal distance and when used within a broad spectral range.
It is known in the art (and can be verified from the hereinafter presented Table 1) that the overall length and sphero-chromatism of high speed objectives cannot be reduced at the same time, and this limit may not be overcome by known means.
The overall length essentially depends on the air separation d.sub.2 between front lens and main mirror lens, as well as the thickness d.sub.1 of the front lens. Sphero-chromatism .DELTA.s'.sub..lambda.'d for light wavelength .lambda. with reference to the fundamental spectral line d is given by EQU .DELTA.s'.sub..lambda.,d =(S'.sub.R -s'.sub.o).sub..lambda. -(s'.sub.R -s'.sub.o).sub.d,
s'.sub.R being the back focal distance of the aperture limited ray and s'.sub.o the back focal distance of the paraxial ray.
A reduction of the aforesaid air separation d.sub.2 would result in a shorter, lighter and above all in more handy objective. This would result in a significant progress over the state of the art in view of the fact that overall length is proportional to the focal length and that objectives of this kind are built generally with a great focal length.
As mentioned above, nevertheless, a reduction in overall length up to now has meant poor sphero-chromatism, thus limiting or even excluding use of such objectives within a broad spectral range or respectively excluding their construction with great focal length.